1. Technical Field
This invention relates to photonic band structures, and more particularly to photonic band structure laser resonator mirrors for improving the far field pattern of diode laser arrays and broad area devices.
2. Discussion
Semiconductor diode laser arrays and broad-area devices are useful in applications where a high output is required as compared to the power available from a single coherent diode laser. Such applications include optical computing, multichannel optical interconnects, free space optical communication systems, laser printers, etc. Coherent single element diode lasers are currently limited in power to outputs on the order of 30 milliwatts (mW). On the other hand, arrays of single diode lasers or lasers with a single broad lasing area can be designed to provide output powers of hundreds or even thousands of milliwatts. The difficulty is to make them spatially and temporally coherent as well.
It is well known that arrays of laser emitters can oscillate in one or more of several possible configurations known as array modes. Similarly, broad-area devices can oscillate simultaneously in one or more of the transverse and longitudinal modes of the associated laser resonator. In the most desirable array mode, all of the emitters oscillate in-phase. This is known as the zero-degree-phase-shift array mode, and it produces a far-field pattern in which most of the energy is concentrated in a single lobe, the width of which is diffraction-limited. Such a diffraction-limited beam is one whose angular spread is limited only by the diffraction of light to a value roughly equal to the wavelength of the emitted light divided by the width of the emitting source. Correspondingly, the most desirable broad-area mode is the one whose wavefront is most uniform in phase. This mode is analogous to the zero-degree-phase-shift array mode and also produces a single lobe diffraction-limited beam.
When adjacent laser emitters are 180 degrees out of phase, the array operates in the 180-degree-phase-shift array mode and produces two off-axis lobes in the far-field pattern of a one-dimensional array and four off-axis lobes in the far-field pattern of a two-dimensional array. The lobes are separated by an angle whose tangent is equal to the wavelength divided by the near field interelement spacing. There are other array modes with far-field patterns between these extremes. Broad-area modes have similar characteristics. Many laser arrays and broad-area devices operate, particularly at higher power, in two or three array modes or transverse modes simultaneously and produce one or more beams that are typically two or three times wider than the diffraction limit. Although various configurations have been proposed to obtain high output power laser arrays, as well as diffraction limited beams, they all have significant limitations, either in their operating characteristics or in the complexity and cost of their fabrication.
Thus it would be desirable to provide a laser diode array or broad-area device which operates at relatively high power and which produces a single-mode diffraction-limited beam. It would also be desirable to provide a laser diode array or broad-area device which couples the laser diodes to operate in a zero-phase-shift array mode to produce a single-main-lobe far-field pattern or which operates in the uniform phase transverse mode. It would also be desirable to provide a laser diode array or broad-area device which is simple and inexpensive to produce.